Transient Alerts — HabitableZone

Transient Alerts

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In plain words Notable

On 15 October 2026 the Einstein Probe, a space telescope that watches for X-ray flashes, saw a brief flare lasting about two minutes in the southern sky. Telescopes then found a faint, slowly changing point of visible light in the same area, sitting on top of a faint galaxy. It faded a little in the first day and a half, then brightened again around day 4 and stayed roughly steady through day 5. A separate, fainter X-ray source in the same region was found to belong to an unrelated, known quasar (a distant galaxy with a very bright core). The leading idea is that the visible light is a supernova (an exploding star) linked to the X-ray flash, but this is not confirmed, and no distance has been measured yet.

How unusual is it? A visible-light counterpart was found for an X-ray flash, and its rebrightening hints at an accompanying supernova, which is not typical of most such flashes. It is not exceptional because the link is unconfirmed, the source could be an unrelated transient, and no distance is known; a spectrum confirming a supernova or giving a distance would settle the verdict.

Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 4); last updated . It may contain mistakes: the reports themselves are the authority.

EP261015a — X-ray Transient, Optical Transient
GCN Circulars —

GCN Circular 45955
EP261015a (WXT 11900947714): Possible optical counterpart from NOT and LCO
From: Gregory Corcoran at University College Dublin
Issued: 2026-10-16 05:15 UTC
Type: X-ray Transient, Optical Transient
Why this event is posted: it has drawn 11 GCN Circulars, at or above the top-10% threshold of 11 for X-ray transients.

G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), P. G. Jonker (Radboud), J. Chacon (PUC), and F. E. Bauer (SSI and UTA), O. Stalder (NOT) report on behalf of a larger collaboration:

We observed the field of the Einstein Probe fast X-ray transient EP261015a (WXT trigger 11900947714) using the ALFOSC camera on the Nordic Optical Telescope (NOT). Observations consisted of a sequence of 3 x 300 s exposures in the SDSS r-band, and started on 2026-10-16 at 02:58:58 UT (13.72 hr after the EP/WXT trigger).

After performing image subtraction against Dark Energy Survey stacked images, we identify a potential counterpart at coordinates (J2000):

RA: 02:57:19.83
Dec: -13:21:17.3

We measure a difference magnitude of r = 22.74 +/- 0.21 (AB) at a midtime of 2026-10-16 03:06:55 UT (13.86 hr after the EP/WXT trigger), calibrated using nearby stars in the Pan-STARRS catalog and not corrected for Galactic extinction.

At a similar mid-time (2026-10-16 03:02:53 UT), we performed additional observations using an LCO 1m telescope located at the Cerro Tololo Interamerican Observatory (Chile) equipped with the SINISTRO instrument. Here observations consisted of 9 x 300 s exposures in SDSS r-band. We see the above candidate source in our difference image as well, at r = 22.83 +/- 0.16 (AB).

We caution it is possible that this source is an artefact of the image subtraction due to presence of an underlying galaxy, however the source’s detection from both telescopes and clean residuals in the photometry lend credence to it being real. Given our two observations are at similar times, we cannot make a useful estimate of its temporal evolution.

Filter: only the top 10% of the past year's X-ray transients are posted, ranked by the number of GCN Circulars written about them (now ≥ 11). Thresholds are recalculated monthly. Only events first reported within the past 7 days can start a thread.
Relayed automatically from NASA's General Coordinates Network.

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